Steel

ABSTRACT

Steel according to one embodiment of the present invention has predetermined chemical components, wherein in a region where a distance r from the center of a cross-section perpendicular to the length direction satisfies 0.7R≤r≤0.9R, structures include ferrite and bainite, the average fraction of the ferrite is in the range of 40 to 70% in terms of area ratio, the total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and the balance includes bainite; and the standard deviation of a ferrite fraction in the region is 4% or less.

TECHNICAL FIELD

The present invention relates to steel.

The present application claims priority to Japanese Patent Application No. 2018-056867 filed in Japan on Mar. 23, 2018, the contents of which are incorporated herein by reference.

BACKGROUND ART

In order to simultaneously achieve precise dimensional accuracy and strength, gears for use in automobiles, construction machinery, industrial machinery, and the like are generally used after receiving carburizing hardening after machining In recent years, quietness during operation has been more strongly demanded than before, and thus an increase in the dimensional accuracy of gears, especially the dimensional accuracy of teeth, has been demanded. The dimensional accuracy of gear teeth is attributable to deformation associated with heat treatment during carburizing hardening (hereinafter referred to as thermal strain). Since this thermal strain varies and is not uniform for each gear tooth, a symmetrical shape is lost in the same gear, and thus vibrations are created during use, thus losing quietness. Accordingly, demands exist for stabilizing thermal strain on gear teeth so as to provide a symmetrical shape.

Concerning conventional technical development of steel for carburized gears, Patent Document 1 discloses a technology for providing steel having excellent cold forgeability and temper softening resistance. However, Patent Document 1 does not provide a technology for stabilizing thermal strain on gear teeth during carburizing hardening, which is an object of the present invention.

Patent Document 2 discloses a technology for providing a hot-rolled steel bar or wire rod composed of a ferrite-pearlite structure, ferrite-pearlite-bainite structure, or ferrite-bainite structure, wherein the standard deviation of ferrite fractions at the time when randomly selected 15 viewing fields of a transverse cross-section are observed and measured with the area per one viewing field being 62500 μm² is 0.10 or less; and when a region from the surface to one-fifth of the radius and a region from the center to one-fifth of the radius in the transverse cross-section are observed, in each of the regions, the amount of Al precipitating as AlN is 0.005% or less, and the density in terms of the number of AlN having a diameter of 100 nm or larger is 5/100 μm² or less. However, in reference to the Examples disclosed in Patent Document 2, it is presumed that the technology of Patent Document 2 uses a pearlite structure to reduce the standard deviation of ferrite fractions. That is to say, according to the technology of Patent Document 2, it is not possible to sufficiently reduce the standard deviation of ferrite fractions while controlling the structures so as not to substantially include pearlite.

CITATION LIST Patent Documents

Patent Document 1: PCT International Publication No. WO 2014/171472

Patent Document 2: PCT International Publication No. WO 2011/055651

SUMMARY OF INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide steel that stabilizes thermal strain on gear teeth during carburizing hardening.

Means for Solving the Problem

The gist of the present invention is as follows.

(1) A steel according to one embodiment of the present invention includes in % by mass:

C: 0.17 to 0.21%,

Si: 0.40 to 0.60%,

Mn: 0.25 to 0.50%,

Cr: 1.35 to 1.55%,

Mo: 0.20 to 0.40%,

S: 0.010 to 0.05%,

N: 0.005 to 0.020%,

Al: 0.001% to 0.100%,

Nb: 0.001 to 0.030%

Ni: 0 to 3.0%,

Cu: 0 to 1.0%,

Co: 0 to 3.0%,

W: 0 to 1.0%,

V: 0 to 0.3%,

Ti: 0 to 0.3%,

B: 0 to 0.005%

O: 0.005% or less,

P: 0.03% or less,

Pb: 0 to 0.5%,

Bi: 0 to 0.5%,

Ca: 0 to 0.01%,

Mg: 0 to 0.01%,

Zr: 0 to 0.05%,

Te: 0 to 0.1%,

rare earth element: 0 to 0.005%, and

a balance including Fe and impurities, wherein

in a region where a distance r from the center of a cross-section perpendicular to a length direction satisfies the following expression, structures include ferrite and bainite, an average fraction of the ferrite is in a range of 40 to 70% in terms of area ratio, a total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and a balance includes bainite; and

a standard deviation of a ferrite fraction in the region is 4% or less:

0.7R≤r≤0.9R

where R represents a circle equivalent radius of the steel.

(2) The steel according to (1) may contain in % by mass one or two or more of:

Ni: 0.01 to 3.0%,

Cu: 0.01 to 1.0%,

Co: 0.01 to 3.0%,

W: 0.01 to 1.0%,

V: 0.01 to 0.3%,

Ti: 0.001 to 0.3%, and

B: 0.0001 to 0.005%.

(3) The steel according to (1) or (2) may contain in % by mass one or two or more of:

Pb: 0.01 to 0.5%,

Bi: 0.0001 to 0.5%,

Ca: 0.0001 to 0.01%,

Mg: 0.0001 to 0.01%,

Zr: 0.0001 to 0.05%,

Te: 0.0001 to 0.1%, and

Rare earth element: 0.0001 to 0.005%.

Effects of the Invention

By using steel of the present invention, thermal strain on the teeth of a gear manufactured by carburizing hardening can be stabilized.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of steel for explaining the positions for measuring the average ferrite fraction and the standard deviation of the ferrite fraction.

EMBODIMENT OF THE INVENTION

Below, embodiments for carrying out the present invention are described in detail.

First, the circumstances of arriving at the present invention are described.

The present inventors have conducted diligent research on a method for stabilizing thermal strain on the teeth of a gear after carburizing hardening. As a result, it was found that the thermal strain is stabilized by increasing the uniformity of structures in a region of steel that becomes teeth after machining. Accordingly, concerning a method for making uniform the structures of a region corresponding to the gear teeth in steel, the inventors further investigated the effect of changing the chemical components of steel and the manufacturing method. As a result, it was found that by configuring the steel components to be in predetermined ranges and then controlling the casting method and the post-rolling cooling rate, the structures of a region corresponding to the gear teeth in steel can be uniform. Concerning the control of the casting method, the cross-sectional area of casting, the casting rate, and the average cooling rate of the surface from the beginning of casting to the correction point are controlled in a combined manner. This makes it possible to homogenize the cast structures of a region in a bloom that eventually becomes gear teeth. Moreover, concerning the control of the post-rolling cooling rate, the cooling rate of the steel surface is controlled. This makes it possible to homogenize the structures of a region in steel that corresponds to the gear teeth.

Next, the reason for limiting the chemical components of steel according to the present embodiment is described. Below, “% by mass” which is a unit relating to the amount of an alloying element is simply referred to as “%”.

C: 0.17 to 0.21%

The C content affects the hardness of the non-carburized portion of a gear. In order to ensure a required hardness, the C content is 0.17% or more. On the other hand, when the C content is excessive, the hardness of the non-carburized portion after carburization is high, resulting in poor impact strength, and thus the C content is 0.21% or less. The preferable lower limit of the C content is 0.175%, 0.18%, 0.185%, or 0.19%. The preferable upper limit of the C content is 0.205%, 0.200%, 0.195%, or 0.19%.

Si: 0.40 to 0.60%

Si is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel. When the Si content is excessive, the amount of ferrite in steel is insufficient, and the amounts of bainite and the like are increased, resulting in poor workability. In order to obtain the above-described effect, the Si content needs to be within the range of 0.40 to 0.60%. The preferable lower limit of the Si content is 0.42%, 0.45%, 0.48%, or 0.50%. The preferable upper limit of the Si content is 0.58%, 0.55%, 0.53%, or 0.51%.

Mn: 0.25 to 0.50%

Mn is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel. In order to obtain the above-described effect, the Mn content needs to be 0.25% or more. When the Mn content is excessive, the amount of ferrite in steel is insufficient, and the amounts of bainite and the like are increased, resulting in poor workability. Accordingly, the Mn content is 0.50% or less. The preferable lower limit of the Mn content is 0.27%, 0.30%, 0.32%, or 0.35%. The preferable upper limit of the Mn content is 0.48%, 0.45%, 0.42%, or 0.40%.

Cr: 1.35 to 1.55%

Cr is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel. When the Cr content is excessive, the amount of ferrite in steel is insufficient, and the amounts of bainite and the like are increased, resulting in poor workability. In order to obtain the above-described effect, the Cr content needs to be within the range of 1.35 to 1.55%. The preferable lower limit of the Cr content is 1.37%, 1.40%, 1.42%, or 1.45%. The preferable upper limit of the Cr content is 1.53%, 1.50%, 1.49%, or 1.47%.

Mo: 0.20 to 0.40%

Mo is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel. When Mo is contained in steel together with Nb, which will be described below, Mo suppresses pearlite transformation by increasing the hardenability of steel, and also suppresses coarse austenite crystal grains during the heating of steel. This makes it possible to suitably control hardenability, and obtain the desired bainite structure by suppressing martensite transformation. When the Mo content is excessive, the amount of ferrite in steel is insufficient, and the amounts of bainite and the like are increased, resulting in poor workability. In order to obtain the above-described effect, the Mo content needs to be within the range of 0.20 to 0.40%. The preferable lower limit of the Mo content is 0.22%, 0.25%, 0.28%, or 0.30%. The preferable upper limit of the Mo content is 0.38%, 0.35%, 0.32%, or 0.30%.

S: 0.010 to 0.05%

S forms MnS in steel, thereby increasing the machinability of steel. In order to obtain a level of machinability that enables components to be cut, a S content comparable to that of commonly used steel for machine structural use is needed. For the above reason, the S content needs to be within the range of 0.010 to 0.05%. The preferable lower limit of the S content is 0.012%, 0.014%, 0.020%, or 0.022%. The preferable upper limit of the S content is 0.035%, 0.030%, 0.028%, or 0.025%.

N: 0.005 to 0.020%

N has a crystal grain refining effect by forming compounds with Al, Ti, V, Cr, and the like, and thus needs to be contained in an amount of 0.005% or more. However, when N exceeds 0.020%, compounds are coarse, and the crystal grain refining effect cannot be obtained. For the above reason, the N content needs to be within the range of 0.005 to 0.020%. The preferable lower limit of the N content is 0.0055%, 0.0060%, 0.007%, or 0.010%. The preferable upper limit of the N content is 0.018%, 0.015%, 0.012%, or 0.010%.

Al: 0.001% to 0.100%

Al is an element effective for the deoxidation of steel, and is an element that binds to N to form nitride and refine crystal grains. When the Al content is less than 0.001%, this effect is insufficient. On the other hand, when the Al content exceeds 0.100%, nitride is coarse and causes embrittlement. The preferable lower limit of the Al content is 0.004%, 0.007%, 0.010%, or 0.020%. The preferable upper limit of the Al content is 0.080%, 0.050%, 0.040%, or 0.030%.

Nb: 0.001 to 0.030%

Nb is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect. Also, Nb when contained in steel together with Mo exerts the above-described synergistic effect (the effect of suppressing pearlite transformation and martensite transformation). When the Nb content is less than 0.001%, this effect is insufficient. When the Nb content exceeds 0.030%, carbonitride is coarse, and this effect cannot be sufficiently obtained. For the above reason, the Nb content needs to be 0.001 to 0.030%. The preferable lower limit of the Nb content is 0.005%, 0.010%, 0.012%, or 0.015%. The preferable upper limit of the Nb content is 0.028%, 0.025%, 0.022%, or 0.020%.

O: 0.005% or Less

O forms oxide in steel and acts as an inclusion to reduce fatigue strength, and thus the O content is preferably limited to 0.005% or less. The preferable upper limit of the O content is 0.003%, 0.002%, 0.0015%, or 0.001%. Since a smaller O content is more preferable, the lower limit of the O content is 0%. However, removing O more than necessary results in increased manufacturing costs. Accordingly, the lower limit of the O content may be 0.0001%, 0.0002%, 0.0005%, or 0.0008%.

P: 0.03% or Less

P segregates at austenite grain boundaries during heating before hardening, thereby reducing fatigue strength. Accordingly, the P content is preferably limited to 0.03% or less. The preferable upper limit of the P content is 0.025%, 0.023%, 0.020%, or 0.015%. Since a smaller P content is more preferable, the lower limit of the P content is 0%. However, removing P more than necessary results in increased manufacturing costs. Accordingly, the substantial lower limit of the P content is usually about 0.004% or more. The lower limit of the P content may be 0.005%, 0.007%, 0.010%, or 0.012%.

Steel according to the present embodiment may further contain one or two or more selected from the group consisting of Ni, Cu, Co, W, V, Ti, and B in place of a part of Fe in order to increase hardenability or the crystal grain refining effect. The lower limit when these elements are not contained is 0%.

Ni: 0 to 3.0%

Ni is an element effective for imparting necessary hardenability to steel. In order to obtain this effect, the Ni content is preferably 0.01% or more. When the Ni content exceeds 3.0%, the amount of residual austenite after hardening is large, resulting in poor hardness. For the above reason, the Ni content is 3.0% or less and more preferably 0.01 to 3.0%. The upper limit of the Ni content is more preferably 2.0% and even more preferably 1.8%. A more preferable lower limit of the Ni content is 0.1% and more preferably 0.3%.

Cu: 0 to 1.0%

Cu is an element effective for increasing the hardenability of steel. In order to obtain this effect, the Cu content is preferably 0.01% or more. When the Cu content exceeds 1.0%, hot ductility is impaired. Accordingly, the Cu content is 1.0% or less and more preferably 0.01 to 1.0%. When obtaining the above-described effect by containing Cu, a more preferable lower limit of the Cu content is 0.05% and even more preferably 0.1%.

Co: 0 to 3.0%

Co is an element effective for increasing the hardenability of steel. In order to obtain this effect, the Co content is preferably 0.01% or more. When the Co content exceeds 3.0%, the effect is saturated. Accordingly, the Co content is 3.0% or less and more preferably 0.01 to 3.0%. When obtaining the above-described effect by containing Co, a more preferable lower limit of the Co content is 0.05% and even more preferably 0.1%.

W: 0 to 1.0%

W is an element effective for increasing the hardenability of steel. In order to obtain this effect, the W content is preferably 0.01% or more. When the W content exceeds 1.0%, the effect is saturated. Accordingly, the W content is 1.0% or less and more preferably 0.01 to 1.0%. When obtaining the above-described effect by containing W, a more preferable lower limit of the W content is 0.05% and even more preferably 0.1%.

V: 0 to 0.3%

V is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect. In order to obtain this effect, the V content is preferably 0.01% or more. When the V content exceeds 0.3%, compounds are coarse, and the crystal grain refining effect cannot be obtained. Accordingly, the V content is 0.3% or less and more preferably 0.01 to 0.3%. When obtaining the above-described effect by containing V, a more preferable lower limit of the V content is 0.1% and even more preferably 0.15%.

Ti: 0 to 0.3%

Ti is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect. In order to obtain this effect, the Ti content is preferably 0.001% or more. When the Ti content exceeds 0.3%, the effect is saturated. For the above reason, the Ti content is 0.3% or less and more preferably 0.001 to 0.3%. A more preferable upper limit of the Ti content is 0.25% and even more preferably 0.2%.

B: 0 to 0.005%

B functions to suppress the grain boundary segregation of P. B also has the effect of increasing grain boundary strength and intragranular strength and the effect of increasing hardenability, and these effects increase the fatigue strength of steel. In order to obtain this effect, the B content is preferably 0.0001% or more. When the B content exceeds 0.005%, the effect is saturated. For the above reason, the B content is 0.005% or less and preferably 0.0001 to 0.005%. A more preferable upper limit of the B content is 0.0045% and even more preferably 0.004%.

The chemical composition of steel according to the present embodiment may further contain one or two or more selected from the group consisting of Pb, Bi, Ca, Mg, Zr, Te, and rare earth elements (REMs) in place of a part of Fe. The lower limit when these elements are not contained is 0%.

Pb: 0 to 0.5%

Pb is an element that increases machinability by being molten and embrittled during cutting. In order to obtain this effect, the Pb content is preferably 0.01% or more. On the other hand, Pb when excessively contained impairs productivity. Accordingly, the Pb content is 0.5% or less and more preferably 0.01 to 0.5%. When obtaining the above-described effect by containing Pb, a more preferable lower limit of the Pb content is 0.05% and even more preferably 0.1%. The preferable upper limit of Pb is 0.4% and even more preferably 0.3%.

Bi: 0 to 0.5%

Bi is an element that increases machinability due to finely dispersed sulfide. In order to obtain this effect, the Bi content is preferably 0.0001% or more. On the other hand, when Bi is excessively contained, the hot workability of steel deteriorates, making hot rolling difficult, and thus the Bi content is 0.5% and more preferably 0.0001 to 0.5%. When obtaining the above-described effect by containing Bi, a more preferable lower limit is 0.0001% and even more preferably 0.001%. The preferable upper limit of Bi is 0.4% and even more preferably 0.3%.

Ca: 0 to 0.01%

Ca is an element effective for the deoxidation of steel and reduces the Al₂O₃ content in oxide. In order to obtain this effect, the Ca content is preferably 0.0001% or more. When the Ca content exceeds 0.01%, a large amount of Ca-containing coarse oxide appears and causes a shortened rolling fatigue life. For the above reason, the Ca content needs to be within the range of 0.0001 to 0.01%. The preferable lower limit of the Ca content is 0.0003% and more preferably 0.0005%. The preferable upper limit of the Ca content is 0.008% and more preferably 0.006%.

Mg: 0 to 0.01%

Mg is a deoxidizing element and produces oxide in steel. Moreover, Mg-based oxide formed by Mg likely becomes a nucleus for crystallization and/or precipitation of MnS. Also, the sulfide of Mg makes MnS spherical by becoming a complex sulfide of Mn and Mg. Thus, Mg is an element effective for controlling the dispersion of MnS and improving machinability. In order to obtain this effect, the Mg content is preferably 0.0001% or more. However, when the Mg content exceeds 0.01%, a large amount of MgS is produced, and the machinability of steel decreases. Thus, in order to obtain the above-described effect by containing Mg, the Mg content needs to be 0.01% or less. The preferable upper limit of the Mg content is 0.008% and more preferably 0.006%. The preferable lower limit of the Mg content is 0.0005% and more preferably 0.001%.

Zr: 0 to 0.05%

Zr is a deoxidizing element and forms oxide. Moreover, Zr-based oxide formed by Zr likely becomes a nucleus for crystallization and/or precipitation of MnS. Thus, Zr is an element effective for controlling the dispersion of MnS and improving machinability. In order to obtain this effect, the Zr content is preferably 0.0001% or more. However, when the amount of Zr exceeds 0.05%, the effect is saturated. Thus, in order to obtain the above-described effect by containing Zr, the Zr content is 0.05% or less and more preferably 0.0001 to 0.05%. The preferable upper limit of the Zr content is 0.04% and more preferably 0.03%. The preferable lower limit of the Zr content is 0.0005% and more preferably 0.001%.

Te: 0 to 0.1%

Te promotes the spheroidization of MnS and thus improves the machinability of steel. In order to obtain this effect, the Te content is preferably 0.0001% or more. When the Te content exceeds 0.1%, the effect is saturated. Accordingly, the Te content is 0.1% or less and more preferably 0.0001 to 0.1%. The preferable upper limit of the Te content is 0.08% and more preferably 0.06%. The preferable lower limit of the Te content is 0.0005% and more preferably 0.001%.

Rare Earth Element: 0 to 0.005%

Rare earth elements are elements that promote the production of MnS by producing sulfide in steel and this sulfide becoming a precipitation nucleus for MnS, and improve the machinability of steel. In order to obtain this effect, the total amount of rare earth elements is preferably 0.0001% or more. However, when the total amount of rare earth elements exceeds 0.005%, sulfide is coarse, reducing the fatigue strength of steel. Accordingly, the total amount of rare earth elements is 0.005% or less and more preferably 0.0001 to 0.005%. The preferable upper limit of the total amount of rare earth elements is 0.004% and more preferably 0.003%. The preferable lower limit of the total amount of rare earth elements is 0.0005% and more preferably 0.001%.

The rare earth element as used herein is a collective term referring to 17 elements including 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 in addition to yttrium (Y) and scandium (Sc) in the periodic table. The amount of rare earth elements means the total amount of one or two or more of these elements.

Steel according to the present embodiment contains the above-described alloying components, and the balance includes Fe and impurities. Elements other than the above-described alloying components are allowable in steel as impurities from raw materials and manufacturing equipment as long as the amounts thereof do not affect the properties of steel.

Next, the uniformity of the structures of steel is described.

As described above, in order to improve the thermal strain on gear teeth, the uniformity of the structures of a region in steel that corresponds to the gear teeth needs to be increased. Here, the region of steel corresponding to the gear teeth is a region including a part from the tooth tip to the tooth root of a gear after forging or cutting, and is a region satisfying 0.7R≤r≤0.9R in rolled steel, wherein r is the distance from the center of the cross-section of steel that is perpendicular to the length direction, and R is a circle equivalent radius in the cross-section of steel that is perpendicular to the length direction of steel.

As a result of investigations by the inventors, it became clear that the uniform structures suitable for improvement of thermal strain are structures including ferrite and bainite, and that the structure fractions are in suitable ranges. According to the investigations of the relationship between structure fractions and thermal strain, the thermal strain was stabilized when, in the 0.7R≤r≤0.9R region, the average value of the ferrite fraction (average fraction) in terms of area ratio is in the range of 40 to 70%, the total of the average fractions of structures other than ferrite and bainite is 0% or more and 3% or less on average, the balance includes bainite, and the standard deviation of the average ferrite fraction in the 0.7R≤r≤0.9R range is 4% or less, as determined by the measurement method described below. When the structure fractions exceeded the above ranges, the thermal strain was increased. Below, what is simply referred to as a “fraction” with respect to a metal structure means the average value of a structure fraction (unit: area %) in the cross-section of steel determined by the means described below. However, in the “standard deviation of a fraction”, the “fraction” does not mean an average value in the entirety of a cross-section but means the fraction in each measured visual field, as will be described below.

The preferable lower limit of the ferrite fraction is 42% and more preferably 45%. The preferable upper limit of the ferrite fraction is 68% and more preferably 65%. A lower standard deviation of the ferrite fraction in the 0.7R≤r≤0.9R range is more preferable, and thus the lower limit is 0%. The preferable upper limit of the standard deviation of the ferrite fraction in the 0.7R≤r≤0.9R range is 3.5% and more preferably 3%.

In steel according to the present embodiment, “bainite” means, among the structures obtained by heating steel to form an austenite single phase structure and then cooling it to room temperature by continuous cooling, a structure excluding a ferrite structure, a pearlite structure, and a martensite structure, and means a collective term referring to an upper bainite structure, a lower bainite structure, or a mixed structure of an upper bainite structure and a lower bainite structure.

It is not preferable that pearlite is contained in the structures of steel according to the present embodiment because it deteriorates carburizing hardenability. For example, when steel composed of mixed structures of ferrite, pearlite, and bainite is carburizing-hardened, the austenite crystal grain structure in a region corresponding to the teeth becomes non-uniform during heating. Accordingly, deformation after carburizing hardening, i.e., thermal strain, is increased. Thus, the area ratio of pearlite needs to be limited as much as possible. In this regard, the total of structures other than ferrite and bainite is specified to be 0% or more and 3% or less. Generally, a structure wherein the total of structures other than ferrite and bainite is 0% or more and 3% or less is referred to as a “ferrite-bainite structure”. In other words, steel according to the present embodiment is steel having a ferrite-bainite structure.

Next, the method for measuring a structure fraction is described.

As shown in FIG. 1, the points where the circumferences having 0.7R+0.25 mm, 0.8R, and 0.9R−0.25 mm intersect straight lines radially dividing, from the center of the cross-section of steel, the cross-section into eight equal parts (central angle 45°) were regarded as measurement points, and rectangular regions having 0.5 mm×1 mm=0.5 mm² were regarded as measurement regions such that the respective measurement points were at the centers of the rectangles. There are 24 measurement regions. The ferrite fraction and the standard deviation of the ferrite fraction in the 0.7R≤r≤0.9R range were determined by observation using an optical microscope with respect to a sample obtained by mirror-polishing the cross-section of steel and corroding it with nital. Since MnS and the like may exist as structures other than ferrite and bainite, each measurement region of the nital-corroded sample was visually observed, and in each measurement region, the 0.5 mm² area in an image captured at 100 observation magnification (captured at 400 observation magnification when the boundary of structures is unclear) was binarized using image processing software Winroof 2015 so as to have ferrite and bainite as bright regions to derive the area ratios of the bright regions, and thereby the ferrite fraction and the bainite fraction of each measurement region were obtained. When determining the area ratios, the area obtained by excluding the area of non-metallic structures such as MnS from the test area was regarded as an evaluated area, and the respective proportions of the areas of the ferrite structure and the bainite structure relative to the evaluated area were regarded as the area ratios of the ferrite structure and the bainite structure. The average value of the ferrite fraction of the 24 measurement regions was regarded as the ferrite fraction, and the average value of the bainite fraction of the 24 measurement regions was regarded as the bainite fraction. The area ratio of structures other than ferrite and bainite were determined by 100−(Ferrite fraction+Bainite fraction). The standard deviation of the ferrite fraction in the 24 measurement points was regarded as the standard deviation of the ferrite fraction of the 0.7R≤r≤0.9R range.

Next, the cross-sectional area and the casting rate during casting, the cooling rate from casting to the correction point, and the post-rolling cooling rate are described.

In order for the inventors to improve thermal strain on gear teeth, it is necessary to strictly specify the ranges of Si, Cr, Mn, and Mo components of steel as described above, and control the casting method and the cooling method during rolling. Concerning the casting method, it is important to control the temperature change of a region corresponding to the gear teeth during casting. When the casting size changes, the temperature and the cooling rate of this region change even at the same casting rate and the same cooling rate. Thus, as a result of investigating the casting size and the temperature change inside a bloom, it became clear that the extent of segregation in a region corresponding to the gear teeth can be controlled by controlling V×A^(0.5)/C, where V is the casting rate, and the unit is m/min; A is the casting size (the cross-sectional area of the bloom), and the unit is mm²; and C is the average cooling rate of the bloom from immediately after casting to the bending correction point. The average cooling rate of the bloom is a value obtained by dividing the temperature difference between the casting temperature of molten steel and the surface temperature of the bloom at the bending correction point by the time required to reach the correction point from immediately below the mold. The unit is ° C./min. The bending correction point is a position where the shape of the bloom is corrected from a curved shape to a straight shape in curved continuous casting.

In order to suitably control the degree of segregation in a region corresponding to the gear teeth, the range of V×A^(0.5)/C needs to be controlled to 6.0 to 20.0. The preferable lower limit is 6.2 or more and more preferably 6.5 or more. The preferable upper limit is 19.0 or less and more preferably 18.0 or less. It is impossible to actually measure the internal temperature during casting, but the use of this formula enables the internal temperature to be estimated in consideration of the items that can be actually measured and the casting size, thereby enabling cast control of a region corresponding to the gear teeth during casting.

As for post-rolling cooling, it is important to control the average cooling rate when the surface temperature of steel during cooling is between 800° C. and 300° C. A uniform structure can be obtained by controlling the average cooling rate to 0.1 to 1.0° C./sec when the surface temperature of steel is between 800° C. and 300° C., and, moreover, the ferrite fraction can be within a predetermined range. When the average cooling rate exceeds this range, a uniform structure cannot be obtained, and thermal strain is increased. The preferable lower limit of the post-rolling cooling rate is 0.15° C./sec or faster and more preferably 0.2° C./sec or faster. The preferable upper limit of the post-rolling cooling rate is 0.9° C./sec or slower and more preferably 0.8° C./sec or slower.

Preferable manufacturing conditions for steel according to the present embodiment are described.

Molten steel, the chemical components of which have been adjusted in a refining step, is cast using a curved continuous casting machine (a casting step). The mold size, the casting rate, and the cooling rate during casting are controlled as described above, and are desirably in the following ranges from the viewpoint of productivity. The mold size is 30000 mm² or more and 400000 mm² or less, the casting rate is 0.2 m/min or faster and 3.0 m/min or slower, and the cooling rate from casting to the correction point is 4.0° C./min or faster and 100° C./min or slower.

The bloom obtained by the casting step is subjected to bloom rolling to obtain a billet (a bloom-rolling step). In order to securely dissolve the Nb compound, the heating temperature during bloom rolling is desirably 1100° C. or higher. A more preferable heating temperature is 1200° C. or higher. On the other hand, an excessively high heating temperature results in coarse crystal grains, and thus the upper limit of the heating temperature is desirably 1280° C. The bloom-rolling reduction of area is desirably 30% or more and more preferably 40% or more.

In order to configure the above billet to be steel for a carburized gear (a steel bar or a wire rod having a circular cross-section), bar rolling or wire rod rolling is performed. In order to securely dissolve the Nb compound, the heating temperature of bar rolling or wire rod rolling is desirably 1100° C. or higher. A more preferable heating temperature is 1150° C. or higher. On the other hand, an excessively high heating temperature results in coarse crystal grains, and thus the upper limit of the heating temperature is desirably 1250° C. As described above, the post-rolling cooling rate is controlled such that the average cooling rate when the surface temperature of steel is between 800° C. and 300° C. is 0.1 to 1.0° C./sec.

A carburized gear is obtained by performing machining on the above steel to form a gear shape and then performing carburizing hardening and tempering. Here, as a method for forming a gear shape, hot forging, cold forging, cutting, or grindstone processing may be performed. Also, in order to increase workability, normalizing and annealing may be performed. Moreover, these may be combined. As for carburizing hardening, any carburizing method such as gas carburizing and vacuum carburizing can be used. Moreover, carbonitriding may be performed. Any type of gear may be created, such as spur gears, helical gears, bevel gears, external teeth, and internal teeth.

EXAMPLES

Below, the present invention is further described by way of Examples. Concerning molten steel having the chemical components of steel numbers 1 to 23, 25, and 26 shown in Table 1, casting was performed under the conditions shown in No. 1 of Table 2 to obtain blooms. The balance of the chemical components disclosed in Table 1 was iron and impurities, and the blank indicates that the component was intentionally not contained. Thereafter, the blooms were heated to 1250° C. and bloom-rolled to obtain billets having 162 mm per side. These billets were heated to 1200° C. and bar-rolled to regulate the diameter thereof to 40 mm, and then cooled under the conditions shown in No. 1 of Table 2 to obtain steels 1 to 23, 33, and 34. Concerning these steels, the structure fractions such as a ferrite fraction and the standard deviation of the ferrite fraction (variation (%) in ferrite fraction) were determined by the above-described methods. The results are shown in Table 3.

Then, in order to evaluate the thermal strain of a gear, a 30 mm wide spur gear having a module of 2, a number of teeth of 16, an inner diameter of φ18 mm was created by cutting. After the gear was retained in an atmosphere wherein gas carburizing was 925° C. and carbon potential CP was 0.8 for 2 hours, oil hardening was performed at 130° C. Thereafter, tempering was performed at 150° C. Thereafter, the shape measurement in the helix direction at 90° pitch for four teeth per gear was performed on five gears by a gear shape measuring machine, and the difference between the maximum value and the minimum value of the helix deviation thus obtained was regarded as a variation in helix deviation. A variation in helix deviation of 15 μm or less was determined as good thermal strain. The results are shown in Test Nos. 1 to 23, 33, and 34 of Table 3.

Test Nos.1 to 19 of the inventive examples had good thermal strain. As for Test Nos. 20 to 23, 33, and 34 of the comparative examples, good thermal strain was not obtained because the chemical component ranges were outside the scope of the present invention.

Specifically, in Test No. 20, the ferrite fraction was insufficient, and the variation in ferrite fraction was excessive. This is presumably because the amount of Si was excessive.

In Test No. 21, the ferrite fraction was insufficient, and the variation in ferrite fraction was excessive. This is presumably because the amount of Mn was excessive.

In Test No. 22, the ferrite fraction was insufficient, and the variation in ferrite fraction was excessive. This is presumably because the amount of Cr was excessive.

In Test No. 23, the ferrite fraction was insufficient, and the variation in ferrite fraction was excessive. This is presumably because the amount of Mo was excessive.

In Test No. 33, the ferrite fraction was insufficient, and moreover the fractions of structures other than ferrite and bainite were excessive. This is presumably because one of Nb and Mo was not contained in steel, and thus the pearlite production suppressing effect of Nb and Mo was not obtained.

In Test No. 34, the fractions of structures other than ferrite and bainite were excessive. This is presumably because one of Nb and Mo was not contained in steel, and thus the pearlite production suppressing effect of Nb and Mo was not obtained.

In Test Nos. 20 to 23, 33, and 34 described above, any one or more of the ferrite fraction, the fractions of structures other than ferrite and bainite, and the variation in ferrite fraction were outside the scope of the invention, and thus it was not possible to suppress the variation in helix deviation.

Next, molten steels having the chemical components shown in Steel Nos. 1, 3, and 24 of Table 1 were cast under the conditions shown in Production Conditions 1 to 12 of Table 2 to obtain blooms. Thereafter, the blooms were heated to 1250° C. and bloom-rolled to obtain billets having 162 mm per side. These billets were heated to 1200° C., bar-rolled to have a shape (a post-rolling diameter) shown in Production Conditions 1 to 12 of Table 2, and cooled under the cooling conditions shown in the same table to obtain steels 1, 24 to 32, 35, and 36. Concerning these steels, the structure fractions such as a ferrite fraction, the standard deviation of the ferrite fraction (variation (%) in ferrite fraction), and the variation in helix deviation were evaluated by the above-described methods. The results are shown in Test Nos. 1, 24 to 32, 35, and 36 of Table 3. Test No. 32 is a test example corresponding to Production No. 1 of PCT International Publication No. WO 2014/171472.

Test Nos. 1 and 24 to 28 of the inventive examples had good thermal strain. On the other hand, since the production conditions were not desirable in Test Nos. 29 to 32, 35, and 36 of the comparative examples, good thermal strain was not obtained.

Specifically, in Test No. 29, the variation in ferrite fraction was excessive. This is presumably because V×A^(0.5)/C was too large, and thus it was not possible to eliminate segregation. Accordingly, in Test No. 29, it was not possible to suppress the variation in helix deviation.

In Test No. 30, the variation in ferrite fraction was excessive. This is presumably because V×A^(0.5)/C was too small, and thus it was not possible to eliminate segregation. Accordingly, in Test No. 30, it was not possible to suppress the variation in helix deviation.

In Test No. 31, the ferrite fraction was insufficient. This is presumably because the post-rolling cooling rate was too fast, and thus the structure thereof was mostly bainite. Accordingly, in Test No. 31, it was not possible to suppress the variation in helix deviation.

In Test No. 32, the variation in ferrite fraction was excessive. This is presumably because V×A^(0.5)/C was too large, and thus it was not possible to eliminate segregation. Accordingly, in Test No. 32, it was not possible to suppress the variation in helix deviation.

In Test No. 35, the variation in ferrite fraction was excessive. This is presumably because the post-rolling cooling rate was too fast, and thus it was not possible to achieve structural uniformity. Accordingly, in Test No. 35, it was not possible to suppress the variation in helix deviation.

In Test No. 36, the fraction of a structure other than ferrite and bainite was excessive. The structure other than ferrite and bainite was pearlite. This is presumably because V×A^(0.5)/C was too small, thus it was not possible to eliminate segregation, and moreover the post-rolling cooling rate was too small. Accordingly, in Test No. 36, it was not possible to suppress the variation in helix deviation. In Test No. 36, the variation in ferrite fraction was suppressed despite V×A^(0.5)/C being too small This is considered to be because the structure included pearlite. However, pearlite also causes an increased variation in helix deviation, and thus it cannot be said that the steel of Test No. 36 is steel that stabilizes thermal strain.

TABLE 1A Steel Chemical composition (mass %) Balance is Fe and impurities No. C Si Mn Cr Mo S N Al Nb O P 1 0.20 0.51 0.43 1.47 0.32 0.015 0.013 0.032 0.013 0.001 0.015 2 0.17 0.59 0.25 1.35 0.40 0.018 0.011 0.022 0.025 0.001 0.019 3 0.21 0.45 0.50 1.54 0.21 0.010 0.012 0.035 0.002 0.001 0.014 4 0.19 0.51 0.44 1.40 0.38 0.013 0.019 0.001 0.029 0.002 0.010 5 0.20 0.40 0.39 1.41 0.30 0.015 0.015 0.027 0.018 0.001 0.023 6 0.21 0.55 0.41 1.39 0.25 0.014 0.013 0.021 0.022 0.001 0.008 7 0.18 0.41 0.40 1.47 0.29 0.013 0.013 0.039 0.017 0.001 0.012 8 0.19 0.43 0.43 1.42 0.33 0.032 0.010 0.035 0.025 0.001 0.014 9 0.20 0.57 0.48 1.51 0.25 0.019 0.015 0.033 0.008 0.001 0.015 10 0.17 0.50 0.45 1.39 0.21 0.011 0.012 0.028 0.014 0.001 0.012 11 0.19 0.46 0.38 1.44 0.39 0.015 0.005 0.099 0.025 0.001 0.014 12 0.20 0.51 0.40 1.41 0.33 0.014 0.011 0.025 0.024 0.001 0.017 13 0.20 0.44 0.44 1.37 0.31 0.014 0.013 0.029 0.022 0.001 0.011 14 0.19 0.48 0.36 1.43 0.32 0.016 0.012 0.027 0.017 0.001 0.013 15 0.18 0.47 0.41 1.41 0.30 0.013 0.009 0.031 0.011 0.001 0.015 16 0.21 0.56 0.40 1.48 0.25 0.014 0.016 0.030 0.022 0.001 0.014 17 0.18 0.45 0.42 1.41 0.29 0.015 0.015 0.035 0.019 0.001 0.011 18 0.18 0.55 0.49 1.44 0.27 0.014 0.013 0.031 0.016 0.001 0.010 19 0.18 0.52 0.42 1.38 0.28 0.015 0.010 0.025 0.022 0.001 0.015 20 0.19 0.62 0.49 1.52 0.38 0.014 0.011 0.030 0.011 0.001 0.014 21 0.19 0.55 0.55 1.51 0.33 0.013 0.013 0.027 0.012 0.001 0.015 22 0.21 0.42 0.27 1.60 0.21 0.014 0.011 0.025 0.004 0.001 0.017 23 0.18 0.47 0.45 1.51 0.48 0.013 0.011 0.028 0.018 0.001 0.012 24 0.18 0.50 0.44 1.45 0.35 0.015 0.013 0.034 0.020 0.001 0.016 25 0.20 0.41 0.45 1.46 0.38 0.011 0.011 0.031 — 0.001 0.012 26 0.21 0.42 0.49 1.52 — 0.015 0.016 0.024 0.035 0.001 0.011 Underlins indicate values outside of the scope of the present invention. Blank columns indicate elements which are not positively included.

TABLE 1B Steel Chemical composition (mass %) Balance is Fe and impurities No. Ni Cu Co W V Ti B Pb Bi Ca Mg Zr Te REM 1 2 3 4 5 0.50 6 0.21 7 0.21 8 0.29 9 0.15 10 0.05 11 0.02 0.0021 12 0.05 13 0.022 14 0.0025 15 0.0028 16 0.0010 17 0.0015 18 0.0035 19 0.29 0.0022 20 21 22 23 24 0.0009 25 26 Underlins indicate values outside of the scope or the present invention. Blank columns indicate elements which are not positively included.

TABLE 2 Cross-sectional Average cooling rate from Post-rolling Manufacturing area of the Casting rate casting to bending correction Post-rolling cooling rate Test No. Steel No. condition mold (mm²) (m/minute) point (° C./minute) V × (A)^(0.5)/C diameter (mm) (° C./second) 1 1 to 23 1 48400 1.4 40.0 7.7 40 0.5 24 1 2 196000 0.5 11.2 19.7 40 0.5 25 1 3 48400 1.7 60.3 6.2 40 0.4 26 1 4 196000 1.0 28.1 15.7 40 1.0 27 1 5 48400 1.2 30.1 8.8 40 0.1 28 1 6 48400 1.4 40.0 7.7 60 0.4 29 1 7 196000 0.4 8.6 20.6 40 0.5 30 1 8 48400 1.1 41.6 5.8 40 0.4 31 1 9 196000 1.1 30.9 15.7 40 1.2 32 24 10 196000 2.0 43.0 20.6 50 1.0 33 25 1 48400 1.4 40.0 7.7 40 0.5 34 26 1 48400 1.4 40.0 7.7 40 0.5 35 3 11 120000 1.0 36.0 9.6 40 1.5 36 3 12 120000 0.6 45.0 4.6 40 0.08 Underlines indicate values outside of the desireble manufacturing conditions.

TABLE 3 Variation Fraction other Variation in Manufacturing Ferrite in ferrite than ferrite and helix deviation Test No. Steel No. condition Classification fraction (%) fraction bainite (%) (μm) 1  1 1 inventive example 55 2 0 10 2  2 1 inventive example 41 4 0 15 3  3 1 inventive example 70 1 0  5 4  4 1 inventive example 47 2 1  8 5  5 1 inventive example 63 3 0  9 6  6 1 inventive example 54 2 0  8 7  7 1 inventive example 61 2 0 11 8  8 1 inventive example 45 2 3  5 9  9 1 inventive example 51 3 1 10 10 10 1 inventive example 47 1 0 11 11 11 1 inventive example 44 2 0  8 12 12 1 inventive example 65 1 0  7 13 13 1 inventive example 51 2 0  9 14 14 1 inventive example 50 3 0  5 15 15 1 inventive example 48 2 0 11 16 16 1 inventive example 45 2 0 14 17 17 1 inventive example 52 2 0 10 18 18 1 inventive example 48 2 0  8 19 19 1 inventive example 55 3 0 13 20 20 1 comparative example 38 6 0 21 21 21 1 comparative example 30 7 0 25 22 22 1 comparative example 35 5 0 19 23 23 1 comparative example 28 7 0 24 24  1 2 inventive example 53 3 0 12 25  1 3 inventive example 49 2 0  8 26  1 4 inventive example 52 3 0 10 27  1 5 inventive example 56 1 0 11 28  1 6 inventive example 45 2 0  9 29  1 7 comparative example 62 6 1 22 30  1 8 comparative example 51 5 1 29 31  1 9 comparative example 32 3 0 19 32 24 10  comparative example 50 7 0 28 33 25 1 comparative example 35 3 10  20 34 26 1 comparative example 48 4 15  25 35  3 11  comparative example 58 8 2 23 36  4 12  comparative example 49 3 12  24 Underlines indicate values outside of the scope of invention, desirable properties, or desirable manufacturing conditions. 

1. Steel comprising in % by mass: C: 0.17 to 0.21%, Si: 0.40 to 0.60%, Mn: 0.25 to 0.50%, Cr: 1.35 to 1.55%, Mo: 0.20 to 0.40%, S: 0.010 to 0.05%, N: 0.005 to 0.020%, Al: 0.001% to 0.100%, Nb: 0.001 to 0.030% Ni: 0 to 3.0%, Cu: 0 to 1.0%, Co: 0 to 3.0%, W: 0 to 1.0%, V: 0 to 0.3%, Ti: 0 to 0.3%, B: 0 to 0.005% O: 0.005% or less, P: 0.03% or less, Pb: 0 to 0.5%, Bi: 0 to 0.5%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to 0.05%, Te: 0 to 0.1%, rare earth element: 0 to 0.005%, and a balance including Fe and impurities, wherein in a region where a distance r from the center of a cross-section perpendicular to a length direction satisfies the following expression, structures comprise ferrite and bainite, an average fraction of the ferrite is in a range of 40 to 70% in terms of area ratio, a total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and a balance includes bainite; and a standard deviation of a ferrite fraction in the region is 4% or less: 0.7R≤r≤0.9R where R represents a circle equivalent radius of the steel.
 2. The steel according to claim 1, which comprises in % by mass one or two or more of: Ni: 0.01 to 3.0%, Cu: 0.01 to 1.0%, Co: 0.01 to 3.0%, W: 0.01 to 1.0%, V: 0.01 to 0.3%, Ti: 0.001 to 0.3%, and B: 0.0001 to 0.005%.
 3. The steel according to claim 1, which comprises in % by mass one or two or more of: Pb: 0.01 to 0.5%, Bi: 0.0001 to 0.5%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Zr: 0.0001 to 0.05%, Te: 0.0001 to 0.1%, and Rare earth element: 0.0001 to 0.005%.
 4. The steel according to claim 2, which comprises in % by mass one or two or more of: Pb: 0.01 to 0.5%, Bi: 0.0001 to 0.5%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Zr: 0.0001 to 0.05%, Te: 0.0001 to 0.1%, and Rare earth element: 0.0001 to 0.005%. 